I've stumbled upon this amazing footage of the great Ramón Montoya playing guitar with his nephew Carlos Montoya. It's a movie from 1938, but in all these years I don't think anyone in flamenco ever noticed. I checked the complete cast of the film, and it identifies them by name, so there's no question.

Although they're in the background, interesting bits can be gleaned, most notably the fact that one is playing por medio and the other por arriba. I wonder if this was when young Carlos decided it would be more fun to be a soloist...